Electrochromic devices are devices that experience a change in colour due to an electrochemical reaction caused by the application of an electric field. The result is a change in e.g. light transmission properties. Electrochromic devices are utilized in many different applications, ranging from architectural windows, information displays, light filters and modulators, rear-view mirrors, sunroofs and windows in vehicles, eyewear, helmet visors, ski goggles, surfaces with variable thermal emissivity to camouflage surfaces. The change from light to dark or from dark to light can be accomplished extremely energy efficiently.
A typical electrochromic device comprises at least five superimposed layers deposited on one substrate or positioned between two substrates in a joined together configuration. The central part of the five-layer electrochromic stack is an ion conductor, typically an electrolyte. The ion conductor is in contact with an electrochromic film, capable of conducting electrons as well as ions. On the other side of the ion conductor is an electron and ion conducting counter electrode film serving as an ion storage layer. The counter electrode film could also be constituted by an electrochromic film. The central three-layer structure is positioned between electron conducting layers. Such a device is coloured or bleached by applying an external voltage pulse between the electron conducting layers on the two sides of the stack, causing the electrons and ions to move between the electrochromic layer and the counter electrode layer.
A commonly expressed requirement of an electrochromic device is to be able to provide a fast change of the colour in a reliable and durable manner. This puts high demands on the electrolyte that should transport the charge from one side of the device to the other. Large efforts have been made to find suitable electrolytes having attractive properties for electrochromic applications.
In the published U.S. patent application 2006/0072182 or the equivalent published international patent application WO 2006/033545, electrolytes suitable for electrochromic purposes are presented. A wide range of mixtures between a compound having an acid functional group and a basic functional group and an ionisable lithium salt is presented. The mixtures are eutectic, thereby forming fluid electrolytes. The electrolytes are typically used in electrochromic devices having rigid substrates. The substrates are put together with a volume intended for the electrolyte in between, and the edges of that volume are sealed off. By providing vacuum in the volume between the substrates, the fluid eutectic mixture can efficiently be sucked into the volume, filling essentially all parts of it. In particular embodiments, the rigidity of the final electrochromic device is enhanced by adding monomers to the eutectic mixture sucked into the electrolyte volume and polymerizing the monomers in situ into a rigid polymer structure around which the electrolyte is disposed.
In some applications, further requirements can be put on the electrochromic devices. Complex geometrical structures of the electrochromic devices may be needed, or the electrochromic device is preferably using one or two flexible substrates. In such situations, the approach of using vacuum for introducing the electrolyte into the device is troublesome. Alternative manufacturing methods are then available. One approach is to prepare two substrates and laminate them together with the electrolyte in between. In such a process, however, the eutectic electrolyte mixtures according to prior art are directly unsuitable due to their fluid properties.